1. Field of the Invention
This invention relates a diode phase shifter and in particular the invention relates to a high efficiency diode phase shifter employing isolated RF and DC circuits with series PIN diodes.
2. Description of the Prior Art:
PIN diodes are commonly used in microwave switching devices and phase shifters. A PIN diode approximates a microwave open circuit when reverse biased and approximates a microwave short circuit when conducting with DC forward bias current.
FIG. 1 shows a commonly used configuration of a single bit digital PIN diode phase shifter in a coupler 10 employing a pair of .lambda./4 microwave or RF transmission lines 12 and 14. In an ideal coupler all the RF power entering input port A splits and is divided equally between ports B and C, with port D being isolated. The split power components at ports B and C reflect off the diodes D1 and D2, which are both selectively reverse or forward biased by the power supply 16 to be either open or short circuit respectively. The power components reenter the coupler at ports B and C. By elementary coupler theory, which need not be discussed in detail here, the power then recombines and exits the output port D. The transmission phase of the circuit between ports A and D is a two state function of the diode states; i.e. either conduction or non-conduction. It should be understood that the power supply 16 is isolated from the RF circuit by the RF chokes 18 and 20 which are coupled to the respective transmission lines 12 and 14. DC current flows through the chokes 18 and 20, the transmission line 14 and the diodes D1 and D2 to ground G.
The current iD1 and iD2 in the respective diodes D1 and D2 result in considerable control circuit power dissipation. The power required for control of the phase shifter bit is equal to the total diode current iD1+iD2 multiplied by the power supply voltage which is typically about 5 volts. For example, if each diode D1 and D2 requires a 50 ma current to induce a low microwave impedance, a total of 100 ma must be drawn from the 5 volts power supply 16 in order to accomplish the function.
In a radar phase shifter there are several of these diodes circuits per phase shifter, and hundreds to thousands of phase shifters per antenna. The power dissipation due to the control of thousands of diodes is a major design problem in many systems, and can result in DC power losses of hundreds of watts. In some applications, such as in spacecraft, the prime power required the drive thousands of diodes places severe limitations on the system practicality. Likewise, cooling requirements of such systems are often expensive, bulky and heavy, further limiting practical applications, especially in air borne systems.